Cr-bearing heat-resistant steel sheet excellent in workability and method for production thereof

ABSTRACT

A Cr-bearing heat-resistant steel sheet with excellent workability comprising, in mass %, C of 0.001% to 0.010%, Si of 0.01% to 0.60%, Mn of 0.05% to 0.60%, P of 0.01% to 0.04%, S of 0.0005% to 0.0100%, Cr of 14% to 19%, N of 0.001% to 0.020%, Nb of 0.3% to 1.0%, Mo of 0.5% to 2.0% and, as required, one or more of Cu of 0.5% to 3.0%, W of 0.01% to 1.0% and Sn of 0.01% to 1.00%, and/or one or more of Ti of 0.01% to 0.20%, Al of 0.005% to 0.100%, Mg of 0.0002% to 0.0100%, and B of 0.0003% to 0.001%, with the remainder comprising iron and unavoidable impurities, and having an x-ray intensity ratio {111}/({100}+{211}) of 2 or greater in the central region of thickness.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The application is a national phase application of International PatentApplication No. PCT/JP03/15988 filed on Dec. 12, 2003, and whichpublished on Jun. 24, 2003 as International Patent Publication No. WO03/053171. Accordingly, the present application claims priority from theabove-referenced International application under 35 U.S.C. § 365. Inaddition, the present application claims priority from Japanese PatentApplication No. 2002-360567 filed Dec. 12, 2002 under 35 U.S.C. § 119.The entire disclosures of these International and Japanese patentapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a Cr-bearing heat-resistant steel withan excellent workability to be usable, e.g., for a material for anautomotive exhaust system that has high-temperature strength andoxidation resistance.

BACKGROUND INFORMATION

Cr-bearing heat-resistant steel sheets are used for exhaust manifolds,mufflers and other exhaust system members that require high-temperaturestrength and oxidation resistance. As these members are manufactured bypress-forming, the steel sheets should have press formability.

Meanwhile, the service temperature for these members rises year afteryear. To cope with this temperature rise, it has been preferable toenhance the high-temperature strength of the material steel sheets byincreasing the addition of Cr, Mo, Nb and other alloying elements.

The addition of alloying elements by simple manufacturing methods,however, has at times, lowered the workability of material steel sheetsto such a level as to make press forming likely impossible.

Increasing the cold reduction ratio is conducive to effectivelyincreasing the “r” value that is an index of press formability of steelsheets. However, the material steel sheets for such exhaust systemmembers are relatively thick (e.g., between approximately 1.5 mm and 2mm). Therefore, the conventional manufacturing processes that limit thethickness of cold-rolled strip to within a certain range do not permitsecuring sufficient cold reduction ratios.

In order to solve the above-described problem by increasing the “r”value, which is an index of press formability, without impairing thehigh-temperature properties, various studies have been made regardingthe chemical composition and manufacturing method of steel sheets.

Conventionally, the workability of Cr-bearing heat-resistant steels hasbeen improved by adjusting the chemical composition as described in, forexample, Japanese Patent Publication No. 09-279312. However, compositionadjustment alone may not be enough to solve the problems, such as crackscaused by pressing, in thicker materials manufactured with relativelylow reduction ratios.

Japanese Patent Publication No. 2002-30346 describes a method thatspecifies the optimum hot-rolled strip annealing temperature based onthe relationship between the hot-rolling starting and finishingtemperatures, Nb content and annealing temperature. However, thespecification of the hot-rolled strip annealing temperature alone issometimes not enough where there are effects of elements (C, N, Cr, Mo,etc.) that are related to Nb-bearing precipitates.

Japanese Patent Publication No. 08-199235 describes a method thatapplies aging treatment to hot-rolled steel strip for more than onehour. This method, however, has a drawback that commercial manufacturingefficiency is extremely low.

SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide a Cr-bearingheat-resistant steel sheet having workability and a method ofmanufacturing the same by solving certain problems that exist inconventional technologies.

Accordingly, one exemplary embodiment of a Cr-bearing heat-resistantsteel sheet is provided with excellent workability. The sheet mayinclude, in mass %, C of 0.001% to 0.010%, Si of 0.01% to 0.60%, Mn of0.05% to 0.60%, P of 0.01% to 0.04%, S of 0.0005% to 0.0100%, Cr of 14%to 19%, N of 0.001% to 0.020%, Nb of 0.3% to 1.0%, Mo of 0.5% to 2.0%,with the remainder comprising iron and unavoidable impurities, andhaving an x-ray intensity ratio {111}/({100}+{211}) of 2 or greater inthe central region of thickness. The sheet may also include, in mass %,one or more of Cu of 0.5% to 3.0%, W of 0.01% to 1.0% and Sn of 0.01% to1.00%. In addition, the sheet may contain, in mass %, one or more of Tiof 0.01% to 0.20%, Al of 0.005% to 0.100%, Mg of 0.0002% to 0.0100% andB of 0.0003% to 0.001%.

According to another exemplary embodiment of the present invention, amethod for manufacturing Cr-bearing heat-resistant steel sheet withexcellent workability is provided. In this exemplary method, a steelsheet is hot-rolled. Such sheet includes, in mass %, C of 0.001% to0.010%, Si of 0.01% to 0.60%, Mn of 0.05% to 0.60%, P of 0.01% to 0.04%,S of 0.0005% to 0.0100%, Cr of 14% to 19%, N of 0.001% to 0.020%, Nb of0.3% to 1.0%, Mo of 0.5% to 2.0% and, if needed, one or more of Cu of0.5% to 3.0%, W of 0.01% to 1.0% and Sn of 0.01% to 1.00%, and/or, oneor more of Ti of 0.01% to 0.20%, Al of 0.005% to 0.100%, Mg of 0.0002%to 0.0100%, and B of 0.0003% to 0.001%, with the remainder comprisingiron and unavoidable impurities, with a heating temperature of 1000° C.to 1150° C. and a finishing temperature of 600° C. to 800° C. The sheet(e.g., the hot-rolled strip) may be coiled at a temperature that is nothigher than 500° C. The coiled hot-rolled metal sheet/strip can beheated to a temperature of between 900° C. and 1000° C. Further, suchsheet/strip may be cooled to a temperature of 300° C. at a rate of 30°C./sec or faster, with subsequent pickling, cooling and annealing.

According to a further exemplary embodiment of the present invention, amethod for manufacturing Cr-bearing heat-resistant steel sheet withexcellent workability is provided. In this exemplary method, a steelsheet is hot-rolled. Such sheet includes, in mass %, C of 0.001% to0.010%, Si of 0.01% to 0.60%, Mn of 0.05% to 0.60%, P of 0.01% to 0.04%,S of 0.0005% to 0.0100%, Cr of 14% to 19%, N of 0.001% to 0.020%, Nb of0.3% to 1.0%, Mo of 0.5% to 2.0% and, if needed, one or more of Cu of0.5% to 3.0%, W of 0.01% to 1.0% and Sn of 0.01% to 1.00%, and/or, oneor more of Ti of 0.01% to 0.20%, Al of 0.005% to 0.100%, Mg of 0.0002%to 0.0100%, and B of 0.0003% to 0.001%, with the remainder comprisingiron and unavoidable impurities, with a heating temperature of 1000° C.to 1150° C. and a finishing temperature of 600° C. to 800° C. Thehot-rolled metal (or strip) is coiled at a temperature not higher than500° C. The coiled hot-rolled metal sheet/strip is recrystallized, andthe metal strip is maintained at a temperature of 900° C. to 1000° C.for not less than 60 seconds. Further, the metal sheet/strip is cooledto a temperature of 300° C. at a rate of 30° C./sec or faster, withsubsequent pickling, cooling and annealing.

According to a further exemplary embodiment of the present invention, amethod for manufacturing Cr-bearing heat-resistant steel sheet withexcellent workability is provided. In this exemplary method, a steelsheet is hot-rolled. Such sheet includes, in mass %, C of 0.001% to0.010%, Si of 0.01% to 0.60%, Mn of 0.05% to 0.60%, P of 0.01% to 0.04%,S of 0.0005% to 0.0100%, Cr of 14% to 19%, N of 0.001% to 0.020%, Nb of0.3% to 1.0%, Mo of 0.5% to 2.0% and, if needed, one or more of Cu of0.5% to 3.0%, W of 0.01% to 1.0% and Sn of 0.01% to 1.00%, and/or, oneor more of Ti of 0.01% to 0.20%, Al of 0.005% to 0.100%, Mg of 0.0002%to 0.0100%, and B of 0.0003% to 0.001%, with the remainder comprisingiron and unavoidable impurities, with a heating temperature of 1000° C.to 1150° C. and a finishing temperature of 600° C. to 800° C. Thehot-rolled metal (or strip) is coiled at a temperature not higher than500° C. The coiled hot-rolled metal sheet/strip can be maintained at atemperature of 750° C. to 950° C. for 1 hour to 30 hours. Further, themetal sheet/strip may be cooled to a temperature of 300° C. at a rate of30° C./sec or faster, with subsequent pickling, cooling and annealing.

The entire disclosures of all publications referenced above areincorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary relationship between x-ray intensity ratio{111}/({100}+{211}) and “r” value of manufactured steel sheets.

FIG. 2 shows an exemplary relationship between a slab heatingtemperature and the “r” value of manufactured strip.

FIG. 3 shows an exemplary relationship between an annealing condition ofhot-rolled strip and the “r” value of manufactured strip.

FIG. 4 shows the relationship between the annealing condition ofhot-rolled strip and the “r” value of manufactured strip.

DETAILED DESCRIPTION

For the purpose of describing the exemplary embodiments of the presentinvention, “%” is used in below to describe “mass %”.

C generally deteriorates workability and corrosion resistance.Therefore, the smaller the content thereof, the better. This is thereason why the upper limit of the content of C is preferably set at0.010%. The lower limit is preferably set at 0.001% because excessivereduction brings about a refining cost increase. When consideringmanufacturing cost and corrosion resistance, it is preferable to limitthe carbon content to between about 0.002 and 0.005%.

Si, which is sometimes added as a deoxidizing element, is also a solidsolution strengthening element. From the viewpoint of materialproperties, therefore, the smaller the content thereof, the better.Thus, the upper limit is preferably set at 0.60%. To secure goodresistance to oxidation, the lower limit is preferably set at 0.01%.However, a further preferable lower limit may be 0.30% because excessivereduction brings about a refining cost increase. When consideringmaterial properties, the preferable upper limit is 0.50%.

Mn, like Si, is a solid solution strengthening element. From theviewpoint of material properties, therefore, the smaller the contentthereof, the better. Thus, the upper limit is preferably set at 0.60%,whereas the lower limit is preferably set at 0.05% in order to securegood scale adhesion. However, a further preferable lower limit is 0.30%because excessive reduction leads to a refining cost increase. Whenconsidering material properties, the preferable upper limit is 0.50%.

P, like Mn and Si, is a solid solution strengthening element. From theviewpoint of material properties, therefore, the smaller the contentthereof, the better. Therefore, the upper limit is preferably set at0.04%. The lower limit is preferably set at 0.01% because excessivereduction brings about a refining cost increase. When consideringmanufacturing cost and corrosion resistance, a further preferablecontent is between 0.02% and 0.03%.

From the viewpoint of material properties and corrosion resistance, thesmaller the content of S, the better. Thus, the upper limit ispreferably set at 0.0100%, whereas the lower limit is preferably set at0.0005% because excessive reduction brings about a refining costincrease. When considering manufacturing cost and corrosion resistance,a further preferable content is between 0.0020% and 0.0060%.

It is preferable to add Cr of not less than 14% for the improvement ofcorrosion and oxidation resistance. However, addition in excess of 19%deteriorates toughness, manufacturability and material properties ingeneral. So, Cr content is limited between 14% and 19%. A furtherpreferable content to secure good corrosion resistance andhigh-temperature strength is 14% to 18%.

As N, like C, deteriorates workability and corrosion resistance, thesmaller the content thereof, the better. Therefore, the upper limit ispreferably set at 0.020%. The lower limit is preferably set at 0.001%because excessive reduction brings about a refining cost increase. Whenconsidering manufacturing cost, workability and corrosion resistance, afurther preferable content is 0.004% to 0.010%.

From the viewpoint of solid-solution and precipitation strengthening, Nbis preferable for the improvement of high-temperature strength. Nb fixesC and N as carbonitrides and affects the development of recrystallizedaggregate structure, that is, x-ray intensity ratio {111}/({100}+{211})of manufactured strip. As the above-described action of Nb appears whenthe content is not less than about 0.3%, the lower limit is set at about0.3%.

As this invention improves workability by controlling Nb-precipitates(in particular, the Laves phase that comprises intermietallic compoundsconsisting essentially of Fe, Cr, Nb and Mo) before cold-rolling, thereshould be a sufficient quantity of Nb to fix C and N. As, however, theeffect saturates at 1.0%, the upper limit is set at 1.0%. Whenconsidering manufacturing cost and manufacturability, the preferablecontent is about 0.4% to 0.7%.

Mo should be included in heat-resistant steels as an element forincreasing corrosion resistance and controlling high-temperatureoxidation. Mo also forms Laves phase. To improve workability bycontrolling the formation of Laves phase, Mo of not less than 0.5% maybe needed.

If Mo content is lower than 0.5%, the Laves phase preferable fordeveloping the recrystallized aggregate structure does not precipitateand, as a result, does not increase the x-ray intensity ratio{111}/({100}+{211}) of manufactured steel sheets. Therefore, the lowerlimit of Mo content is preferably set at 0.5%.

As, however, excessive addition deteriorates toughness and lowerselongation properties, the upper limit is preferably set at 2.0%. Whenconsidering manufacturing cost and manufacturability, a furtherpreferable content is 1.0% to 1.5%.

Cu is added as required for increasing corrosion resistance andhigh-temperature strength. Addition of Cu of preferably not less than0.5% precipitates ε-Cu and, thereby, increases the x-ray intensity ratio{111}/{100}+{211}). Therefore, the lower limit set at 0.5%.

As, however, excessive addition lowers elongation properties anddeteriorates manufacturability, the upper limit is preferably set at3.0%. When considering manufacturing cost and manufacturability, thepreferable content is preferably 1.0% to 2.0%.

W is added as required for increasing high-temperature strength. As thisaction appears when W of preferably not less than 0.01% is added, thelower limit is preferably set at 0.01%. As, however, excessive additionlowers manufacturability and workability, the upper limit is preferablyset at 1.0%. When considering high-temperature properties andmanufacturing cost, the preferable content is about 0.05% to 0.5%.

Sn is added as required for increasing high-temperature strength andlowers recrystallization temperature by segregating at grain boundaries.As this action appears when Sn of preferably not less than 0.01% isadded, the lower limit is preferably set at 0.01%. As, however,excessive addition deteriorates workability and tends to form surfacedefects during manufacturing, the upper limit is preferably set at1.00%. When considering high-temperature properties and manufacturingcost, a further preferable content is about 0.05% to 0.50%.

Ti is added as being preferable for further improving corrosionresistance, intergranular corrosion resistance and deep drawability bycombining with C, N and S. As the action to increase the x-ray intensityratio {111}/({100}+{211}) appears when the content is preferably notlower than 0.01%, the lower limit is preferably set at 0.01%.

A combined addition of Ti and Nb improves high-temperature strength andcontributes to the improvement of oxidation resistance. However,excessive addition impairs manufacturability in the steelmaking process,induces defect formation in the cold-rolling process and brings aboutdeterioration of material properties by increasing solid solution of Ti.Therefore, the upper limit is preferably set at 0.20%. When consideringmanufacturing cost, a further preferable content is 0.03% to 0.10%.

Al is sometimes added as a deoxidizing element. As the deoxidizingaction appears when the content is not less than 0.005%, the lower limitis preferably set at 0.005%. As addition in excess of 0.100% lowerselongation properties and deteriorates weldability and surface quality,the upper limit is set at 0.100%. When considering a refining cost, afurther preferable content is 0.010% to 0.070%.

Mg forms Mg-oxide in molten steel and acts as a deoxidizing agenttogether with Al. Fine precipitation of Nb— or Ti-precipitates occursaround finely crystallized Mg-oxide. When these precipitates finelyprecipitate in the hot-rolling process, very fine recrystallizedstructures are formed around the fine precipitates, thereby increasingthe x-ray intensity ratio {111}/({100}+{211}) and remarkably improvingthe workability of cold-rolled annealed steel sheets. As this actionappears when the content is preferably not lower than 0.0002%, the lowerlimit is preferably set at 0.0002%.

As, however, excessive addition lowers weldability, the upper limit ispreferably set at 0.0100%. When considering refining cost, a furtherpreferable content is 0.0005% to 0.0020%.

B of not less than 0.0003% is added for improving cold workability andfabricability of manufactured steel sheets. However, addition in excessof 0.001% deteriorates ductility and deep drawability. Therefore, theupper limit is preferably set at 0.001%. A further preferable content is0.0005% to 0.0010%.

Further, the relationship between the x-ray intensity ratio and “r”value is discussed below.

The “r” value, which is an indicator of workability, is related to therecrystallized aggregate structure. Generally, the “r” value improves ifthe ratio of plane direction {111} to {100}, i.e. ({111}/{100}), isincreased. Through an investigation that takes into consideration of theinfluences of other plane directions as well, the inventors discoveredthat plane direction {211} too should be considered for the improvementof the “r” value.

FIG. 1 shows the relationship between the x-ray intensity ratio{111}/({100}+{211}), and mean “r” value in the central region ofthickness of cold-rolled annealed Cr-bearing heat-resistant steel sheet(containing C of 0.003%, Si of 0.5%, Mg of 0.5%, P of 0.02%, S of0.001%, Cr of 14.5%, Nb of 0.6%, Mo of 1.4% and N of 0.01%) that affectscracking during pressing.

The x-ray intensity ratio plotted along the horizontal axis was derivedfrom the x-ray intensity strength measured on different crystal faces inthe central region of thickness of cold-rolled annealed steel sheet andthe intensity ratio of non-oriented steel specimens.

The mean “r” value plotted along the vertical axis may be derived byapplying 15% in the rolling direction and directions 45° and 90° awaytherefrom on JIS 13B tensile test specimens taken from cold-rolledannealed steel sheet and using equations (1) and (2).r=1n(W ₀ /W)/1n(t ₀ /t)   (1)

where W₀ is the sheet width before application of strain, W is the sheetwidth after application of strain, t₀ is the sheet thickness beforeapplication of strain, and t is the sheet thickness after application ofstrain.Mean r value=(r ₀+2r ₄₅ +r ₉₀)/4   (2)

where r₀ is the “r” value in the rolling direction, r₄₅ is the “r” valuein a direction 45 degrees away from the rolling direction, and r₉₀ isthe “r” value in a direction 90 degrees away from the rolling direction.

As shown in FIG. 1, the x-ray intensity ratio {111}/({100}+{211}) and“r” value are in a proportional relationship and, therefore, the “r”value improves as the x-ray intensity ratio {111}/({100}+{211})increases. When the x-ray intensity ratio {111}/({100}+{211}) is 2 orabove (in the range PI in the figure), the mean “r” value is 1.4 orabove, which means that workability is high enough to permit fabricationof general exhaust system members.

It has been determined that controlling the formation of Nb-basedprecipitates improves the “r” value.

FIG. 2 shows exemplary influences of heating and finishing-rollingtemperatures on the “r” value of Cr-bearing heat-resistant steel sheet(containing C of 0.003%, Si of 0.5%, Mn of 0.5%, P of 0.02%, S of0.001%, Cr of 14.5%, Nb of 0.6%, Mo of 1.4% and N of 0.01%) prepared byhot-rolling to a thickness of 5.0 mm with a coiling temperature of 500°C. and annealing temperature of 950° C. and cold-rolling to a thicknessof 1.5 mm with an annealing temperature of 1050° C.

In FIG. 2, the circled numbers designate the mean “r” values. As shownin FIG. 2, “r” values 1.4 or above can be obtained by heating at 1000°C. to 1150° C and finishing-rolling at 600° C. to 800° C. (See hatchedarea in FIG. 2).

If the temperatures are outside the range specified according to theexemplary embodiments of the present invention, appropriate precipitatesmay be unobtainable in the manufacturing process. As a consequence, thex-ray intensity ratio of cold-rolled steel strip is out of thepreferable range and the preferable “r” value may not be obtained.

If the heating temperature is under 1000° C. and/or thefinishing-rolling temperature is under 600° C. (see the area indicatedby arrow in the figure), many surface defects due to seizure withhot-rolling rolls are formed. Such surface defects significantlydeteriorate the surface quality and become the starting point ofcracking during pressing. Therefore, the lower limits of the heating andfinishing-rolling temperatures are respectively set at 1000° C. and 600°C.

One of the reasons why the exemplary embodiments of the presentinvention can improve the “r” value is that fine recrystallization isachieved by implementing hot-rolling at low temperature, increasingstored strain and accelerating recrystallization in the subsequentannealing process. With the chemical composition according to theexemplary embodiments of the present invention, Nb-based precipitatesprecipitate at 1200° C. or below. During hot-rolling, therefore, aworking strain is introduced around the finely precipitated Nb-basedprecipitates in the mother phase.

In order to accumulate strain in hot-rolling, it is preferable toincrease stored strain by coiling the finish-rolled strip at lowtemperature. Therefore, coiling at a low temperature may be. As storedstrain does not recover if the coiling temperature is not higher than500° C., the coiling temperature is preferably set at a temperature ofpreferably not higher than 500° C. As, however, an excessively lowtemperature leads to malformed strip, a further preferable temperatureis about 400° C. to 500° C.

Hot-rolled steel strip is generally annealed for securing desiredproperties by recrystallizing the ferrite structure. The basicmetallurgical principle for improving the “r” value is to refine theferrite structure in hot-rolled annealed steel before cold-rolling,facilitate the introduction of strain from grain boundaries, and developthe crystal orientation (such as {111}<112>) that improves the “r” valueduring annealing of cold-rolled steel sheet.

However, to improve the “r” value by controlling the quantity and sizeof Nb-based precipitates, even without forming recrystallized structureby annealing hot-rolled steel strip.

FIG. 3 shows the relationship between the annealing temperature ofhot-rolled steel strip and the mean “r” value of cold-rolled annealedsteel strip prepared by annealing hot-rolled strip of Cr-bearingheat-resistant steel strip (containing C of 0.003%, Si of 0.5%, Mg of0.5%, P of 0.02%, S of 0.001%, Cr of 14.5%, Nb of 0.6%, Mo of 1.4% and Nof 0.01%) and cold-rolling to 300° C. at a rate of approximately 30°C./sec, with a slab heating temperature of about 1150° C., a coilingtemperature of about 500° C., hot-rolled strip thickness of 5.0 mm,cold-rolled strip thickness of 1.5 mm and cold-rolled strip annealingtemperature of 1050° C.

FIG. 3 shows that the “r” value of the cold-rolled annealed steel stripbecomes 1.4 or higher (see the range PI in the figure) by heating thehot-rolled strip to between 900° C. and 1000° C. and cold-rolling to300° C. at a rate of 30° C./sec.

Though the structure of the hot-rolled steel strip is not recrystallizedin the temperature range between about 900° C. and 1000° C. as therecrystallizing temperature thereof is 1050° C. (see “Tre” in thefigure), the mean “r” value is high. This is because, among the Nb-basedprecipitates (Nb(C,N) and the Laves phase), the Laves phase, inparticular, precipitates in large enough quantity and size to acceleraterecrystallization in the subsequent cold-rolled strip annealing process.

If the temperatures are outside the range specified by the presentinvention (the range PI in the figure), appropriate precipitates areunobtainable in the manufacturing process. As a consequence, the x-rayintensity ratio of cold-rolled steel strip is outside the preferablerange and the preferable “r” value cannot be obtained.

If the hot-rolled steel strip is annealed at a temperature higher than1000° C., much of the Nb-based precipitates becomes a solid solution andre-precipitates when the cold-rolled strip is annealed, therebysignificantly delaying the recrystallization of the ferrite phase andimpeding the growth of the recrystallization orientation that increasesthe “r” value.

If the hot-rolled strip is annealed at a temperature under 900° C., alarge quantity of fine Laves phase not larger than 0.1 μm precipitates.In the subsequent annealing of the cold-rolled steel strip, the fineLaves phase acts as a pin to inhibit recrystallization and significantlydelays the recrystallization of the ferrite phase.

In order to prevent the precipitation of the fine Laves phase duringcold-rolling, the faster the cold-rolling rate, the better. Thepreferable cold-rolling rate is 30° C./sec or faster.

The recrystallizing temperature of the hot-rolled steel strip varieswith the alloy composition. Depending on other properties, it is attimes preferable to recrystallize the hot-rolled steel strip. Theinventors discovered that heating to and holding between 900° C. and1000° C. is effective because heat treatment is done at a temperaturenot lower than the recrystallizing temperature and the Laves phasedescribed earlier is controlled subsequently.

FIG. 4 shows the relationship between the holding time of hot-rolledstrip annealing temperature and the mean “r” value of cold-rolledannealed steel strip prepared by annealing a hot-rolled strip ofCr-bearing heat-resistant steel (containing C of 0.003%, Si of 0.5%, Mnof 0.5%, P of 0.02%, S of 0.001%, Cr of 14.5%, Nb of 0.6%, Mo of 1.4%and N of 0.01%) and cold-rolling to 300° C. at a rate of 30° C./sec,with a slab heating temperature of 1150° C., coiling temperature of 500°C., hot-rolled strip thickness of 5.0 mm, hot-rolled strip heatingtemperature of 1100° C., cold-rolled strip thickness of 1.5 mm andcold-rolled strip annealing temperature of 1050° C.

As shown in FIG. 4, the mean “r” value of not lower than 1.4 is obtainedif the strip is heated to a temperature range between 900° C. and 1000°C. and held in the same range for not shorter than 60 seconds. If thetemperatures are outside the range specified by the present invention(the range PI in the figure), appropriate precipitates are unobtainablein the manufacturing process. As a consequence, the x-ray intensityratio of cold-rolled steel strip is outside the preferable range and thepreferable “r” value cannot be obtained.

Hot-rolled steel strip can be heated to a temperature not lower than therecrystallizing temperature either by continuous annealing that heattreats steel strip continuously or by batch annealing which requireslong time. Heating to the temperature range between about 900° C. and1000° C. can be accomplished either by first heating to therecrystallizing temperature and then reheating after cooling to roomtemperature or by holding in the cold-rolling process after heating tothe recrystallizing temperature. In all these cases, the cold-rollingrate to 300° C. should be not slower than about 30° C./sec for thereason described above.

In order to control the quantity and size of Nb-based precipitates,hot-rolled steel strip can be heat-treated over a long period of time,as described earlier. Particularly if strip is held between 750° C. and950° C. for 1 hour to 30 hours, Nb-precipitates are formed in anappropriate way to contribute to the improvement of workability. Heattreatment can be applied either by batch annealing or by holding theheat during coiling of hot-rolled strip. In view of the productionefficiency, the preferable heat treatment temperature is about 800° C.to 900° C.

Examples of the exemplary embodiments of the present invention aredescribed below. The conditions used in the examples are those whichwere used to demonstrate the practicability and effect of the presentinvention which is by no means limited thereto. The present inventioncan be put into practice under various conditions without departing fromthe spirit and purpose thereof.

EXAMPLE

Steels of chemical compositions listed in Tables 1 and 2 were cast toslab that was then hot-rolled to 5.0 mm thick strip. The hot-rolledstrip was then continuously annealed, pickled, cold-rolled to athickness of 1.5 mm, and then made into finished product by applyingcontinuous annealing and pickling. Tables 3 and 4 shows themanufacturing conditions employed.

Specimens were taken from the finished product described above and thex-ray intensity, “r” value and elongation in the central region ofthickness were measured. The x-ray intensity and “r” value were measuredby the same method as described earlier.

Elongation at break was determined by taking JIS 13B tensile testspecimens from the finished-strip and applying tensile force in therolling direction. If the elongation is under 30%, the finished-stripdoes not withstand stretch forming even if the “r” value is high.Therefore, elongation must not be less than 30%.

TABLE 1 X-ray Elon- intensity Mean ga- ratio “r” tion {111}/ value of({100} + of fin- {211}) of fin- ished Steel finished ished strip, No. CSi Mn P S Cr N Nb Mo Cu W Sn Ti Al Mg B strip strip % 1 0.005 0.53 0.550.03 0.0008 13.9 0.009 0.61 1.4 — — — — — — — 3.0 1.5 35 2 0.003 0.080.07 0.01 0.0001 14.5 0.005 0.58 1.5 — — — — — — — 2.5 1.4 32 3 0.0040.11 0.13 0.01 0.0012 18.8 0.005 0.77 1.5 — — — — — — — 2.6 1.5 31 40.003 0.08 0.07 0.01 0.0001 14.5 0.005 0.83 1.5 — — — — — — — 3.0 1.6 345 0.003 0.49 0.52 0.02 0.0011 14.0 0.009 0.55 1.3 2.5 — — — — — — 4.01.8 32 6 0.006 0.23 0.45 0.01 0.0015 18.5 0.004 0.63 1.5 1.5 0.14 — — —— — 4.2 1.8 31 7 0.008 0.58 0.56 0.04 0.0033 14.1 0.002 0.90 0.5 — —0.05 — — — — 4.1 1.8 33 8 0.007 0.45 0.31 0.02 0.0023 16.8 0.006 0.530.6 0.8 — 0.08 — — — — 3.8 1.7 33 9 0.008 0.50 0.50 0.01 0.0016 14.30.001 0.66 1.1 0.6 0.09 — — — — — 2.8 1.5 32 10 0.009 0.07 0.09 0.010.0010 15.5 0.015 0.35 2.9 — 0.70 0.70 — — — — 2.9 1.6 31 11 0.002 0.070.06 0.03 0.0007 14.6 0.016 0.33 0.6 — — — 0.11 — — 0.0005 3.3 1.7 36 120.007 0.58 0.33 0.01 0.0053 15.8 0.011 0.45 0.7 — — — 0.010 — 4.1 1.8 3513 0.004 0.35 0.25 0.01 0.0025 16.3 0.008 0.56 1.1 — — — — — 0.0002 —4.5 1.9 38 14 0.005 0.26 0.41 0.01 0.0013 17.8 0.013 0.68 1.6 — — — 0.030.07 0.0003 2.5 1.5 35 15 0.006 0.15 0.11 0.02 0.0021 18.6 0.005 0.771.9 — — — 0.18 — 0.0011 — 2.4 1.4 36 16 0.009 0.06 0.09 0.01 0.0015 18.30.003 0.81 1.4 — — — 0.006 0.0005 — 3.9 1.7 35 17 0.006 0.38 0.45 0.040.0009 17.1 0.004 0.93 1.2 0.7 — — 0.02 — — 0.0010 4.5 1.8 35 18 0.0030.21 0.55 0.02 0.0011 16.2 0.001 0.83 1.1 2.8 — — 0.17 0.006 — 0.00083.3 1.6 34 19 0.003 0.13 0.22 0.01 0.0019 15.4 0.013 0.74 0.7 — — — 0.03— 0.0002 0.0005 3.2 1.6 35 20 0.003 0.12 0.39 0.01 0.0038 14.2 0.0180.61 0.6 — 0.05 0.12 0.15 — — 0.0004 2.5 1.5 32 21 0.003 0.02 0.1 0.020.001 16.1 0.011 0.47 1.7 — — — 0.15 0.013 0.0002 0.0008 3.0 1.5 35 220.004 0.11 0.16 0.03 0.0041 14.1 0.004 0.55 0.5 1.4 — — 0.09 — 0.00500.0009 3.1 1.6 34

TABLE 2 Steel No. C Si Mn P S Cr N Nb Mo Cu W Sn 23 0.005 0.53 0.55 0.030.0008 13.9 0.009 0.61 1.4 — — — 24 0.006 0.8* 0.35 0.02 0.0009 14.30.001 0.60 1.3 — — — 25 0.007 0.42 1.2* 0.02 0.0012 14.5 0.001 0.59 1.4— — — 26 0.003 0.55 0.07 0.01 0.0001 14.5 0.005 0.58 1.5 — — — 27 0.0040.11 0.60 0.01 0.0012 18.8 0.005 0.77 1.5 — — — 28 0.003 0.08 0.07 0.05*0.0004 14.5 0.005 0.83 1.5 — — — 29 0.003 0.49 0.52 0.02 0.0015 14.00.009 0.55 1.3 — — — 30 0.005 0.33 0.42 0.03 0.023* 14.1 0.001 0.65 1.5— — — 31 0.006 0.23 0.45 0.01 0.0015 20.5* 0.004 0.63 1.5 — — — 32 0.0080.58 0.56 0.04 0.0033 14.1 0.025* 0.90 0.5 — — — 33 0.007 0.45 0.31 0.020.0023 16.8 0.006 1.3* 0.6 — — — 34 0.009 0.55 0.29 0.03 0.0013 16.50.017 0.25* 1.1 — — — 35 0.007 0.45 0.31 0.02 0.0023 16.8 0.006 0.31 0.6— — — 36 0.008 0.50 0.50 0.01 0.0016 14.3 0.001 0.66 2.4* — — — 37 0.0090.44 0.55 0.03 0.0022 14.5 0.012 0.51 0.4* — — — 38 0.002 0.07 0.06 0.030.0007 14.6 0.016 0.33 0.6 3.8* — — 39 0.005 0.35 0.55 0.03 0.0011 14.10.013 0.41 0.7 0.4* — — 40 0.004 0.35 0.25 0.01 0.0025 16.3 0.008 0.561.1 — 1.5* — 41 0.006 0.15 0.11 0.02 0.0021 18.6 0.005 0.77 1.9 — — 1.5*42 0.005 0.23 0.25 0.02 0.0023 14.5 0.015 0.44 1.5 1.2 — 0.02* 43 0.0060.38 0.45 0.04 0.0009 17.1 0.004 0.93 1.2 — — — 44 0.008 0.22 0.36 0.040.0023 16.9 0.0016 0.65 1.1 — — — 45 0.003 0.13 0.22 0.01 0.0019 15.40.013 0.74 0.7 — — — 46 0.004 0.11 0.16 0.03 0.0041 14.1 0.004 0.55 0.5— — — 47 0.005 0.25 0.25 0.03 0.0035 14.3 0.011 0.45 0.5 — — — 48 0.0030.04 0.1 0.02 0.001 16.1 0.011 0.47 1.7 — — — X-ray intensity ratio Mean“r” {111}/({100} + value of Elongation Steel {211}) of finished offinished No. Ti Al Mg B finished strip strip strip, % 23 — — — — 1.7*1.2* 27* 24 — — — — 2.5 1.4 28* 25 — — — — 2.5 1.3 27* 26 — — — — 1.5*1* 32 27 — — — — 1* 0.9* 33 28 — — — — 2.5 1.4 29* 29 — — — — 1.6* 1.1*34 30 — — — — 2.6 1.5 26* 31 — — — — 1.9* 1.3 28* 32 — — — — 0.5* 0.6*28* 33 — — — — 1.5* 1.1* 24* 34 — — — — 1.6* 1.2* 31 35 — — — — 1.4* 1*32 36 — — — — 1.1* 0.8* 25* 37 — — — — 1.6* 1.2* 32 38 — — — — 2.2 1.5*29* 39 — — — — 1.8* 1.3* 33 40 — — — — 1.4* 1* 23* 41 — — — — 1* 0.8*24* 42 — — — — 1.1* 0.9* 33 43 0.38* — — — 1.8* 1.3* 28* 44 0.005* — — —1.7* 1.3* 32 45 — 0.16* — — 2.1 1.4 29* 46 — — 0.013* — 3.0 1.5 29* 47 —— 0.0001* — 1.9 1.3* 33 48 0.15 0.013 0.0002 0.0021* 1.7* 1.2* 26**Outside the scope of the present invention

TABLE 3 X-ray intensity Mean Elonga- Hot-rolling conditions Hot-rolledstrip annealing conditions ratio “r” tion Heating Finishing CoilingHeating Holding Holding Cold- {111}/({100} + value of of Steeltemperature, temperature, temperature, temperature, temperature, time,rolling rate, {211}) finished finished No. ° C. ° C. ° C. ° C. ° C. sec° C./sec of finished strip strip strip, % inven- 49 1150 790 490 950 non— 30 2.0 1.4 35 tive 50 1090 730 450 950 non — 40 2.2 1.5 36 Ex- 51 1030650 300 910 non — 80 2.3 1.6 35 amples 52 1150 800 450 1080 950 60 403.3 1.8 36 53 1050 780 500 1100 1000 70 30 2.8 1.6 35 54 1020 630 4751050 930 60 50 3.0 1.7 36 55 1150 650 460 950 non — 35 3.0 1.7 32 561100 660 450 1100 950 100 40 3.0 1.7 32 57 1140 730 500 980 non — 40 2.01.4 31 58 1130 750 310 1100 950 120 30 3.1 1.7 33 59 1150 796 350 1020non — 50 2.3 1.5 36 60 1110 710 500 1100 950 180 60 3.2 1.8 36 61 1060630 470 1030 non — 30 2.7 1.6 35 62 1050 620 410 1100 940 60 70 3.2 1.836 63 1030 645 360 930 non — 100 3.1 1.7 35 64 1150 730 425 1100 990 6030 2.7 1.6 34 65 1020 740 430 940 non — 60 2.0 1.4 32 66 1030 625 5001100 930 200 40 3.5 1.9 34 67 1010 635 486 950 non — 80 3.3 1.8 34 681030 680 485 1100 980 100 90 2.0 1.7 33 69 1150 790 490 — 850 21600 50°C./hr 2.0 1.4 35 70 1150 790 490 — 750 108000 40° C./hr 2.2 1.5 36

TABLE 4 X-ray intensity Mean Elonga- Hot-rolling conditions Hot-rolledstrip annealing conditions ratio “r” tion of Heating Finishing CoilingHeating Holding Holding Cold- {111}/({100} + value of finished Steeltemperature, temperature, temperature, temperature, temperature, time,rolling rate, {211}) of finished strip, No. ° C. ° C. ° C. ° C. ° C. sec° C./sec finished strip strip % relative 71  1200* 790 490  950 non — 401.1* 1.1* 34 ex- 72 1150  860* 490 1000 non — 50 1.3* 1.2* 33 amples 731150 790  650* 1100  950  100 60 1.2* 1.2* 35 74 1130 770 490 1050* non— 30 1.1* 1.2* 31 75 1150 750 490 1000 non — 15* 1.3* 1.3* 32 76 1140790 490 1080 1030*  60 30 1* 1* 31 77 1050 720 490 1050  850*  130 20*1.1* 1.2* 30 78 1150 650 500  870* non — 30 0.9* 0.9* 31 79 1160 690 4501100 1050*  200 40 1.2* 1.1* 32 80 1050 800 450 1050* non — 80 1.3* 1.2*31 81 1100 760 480 1080 1020*  300 40 1.2* 1.1* 30 82 1060 780 470 1030*non — 30 1.2* 1.3* 35 83 1030 750 440 1050 1010*  120 50 1* 1* 33 841050 800 500 1100* non — 35 1.2* 1.1* 34 85 1140 630 470 1090 1050*  11020* 1.5* 1.2* 33 86 1150 760 440 1120* non — 40 1.3* 1* 34 87 1130 770420 1100  870*  70 30 0.8* 0.9* 32 88 1100 800 450  770* non — 50 0.5*0.6* 30 89 1100 630 460 1150  830*  300 20* 0.9* 0.9* 32 90 1100 700 4501060* non — 40 1.1* 1.1* 33 91 1100 700 430 1100  750*  160 30 0.6* 0.7*32 92 1150 790 490 —  850 1800* 50° C./hr 1.1* 1.1* 34 93 1150 790 490 — 750 1200* 40° C./hr 1.3* 1.2* 33 *Outside the scope of the presentinvention

Provided below are the findings obtained from Tables 1 and 2. Thefinished-strips manufactured from the steels of the compositionsaccording to the present invention have higher mean “r” values andbetter workability than the strips prepared for comparison. Even ifchemical composition is within the range of the present invention,preferable x-ray intensity is not obtained and, therefore, the “r” valuedoes not improve if the x-ray intensity ratio is outside the range ofthe present invention.

If Si, Mn, P, S, Cu and Ti contents exceed the upper limit thereof, notmany precipitates, that affect the x-ray intensity, are formed.Although, therefore, the x-ray intensity and “r” value are within therange according to the invention, elongation drops significantly becauseof solid solution strengthening and intergranular segregation.

If C and N contents exceed the upper limit thereof, solid solutions of Cand N increase. As a consequence, the desired x-ray intensity is notobtained and elongation drops. Cr, Nb, Mo, Sn and W form intermetalliccompounds and segregate at grain boundaries. If, therefore, theircontents exceed the upper limit specified by the invention, the desiredx-ray intensity and elongation are not obtained because of plentifulprecipitation of fine precipitates and solid solution strengthening.

If Nb and Mo contents fall below the lower limit thereof specified bythe exemplary embodiments of the present invention, sufficientprecipitation of the Laves phase and sufficient fixing of C and N arenot achieved. As a consequence, the x-ray intensity drops and thedesired “r” value is unobtainable. Excessive addition of Mg, though theinfluence on the x-ray intensity is small, makes the precipitates andoxides too coarse and, therefore, brings about a drop in elongation.

Tables 3 and 4 show the influences of manufacturing conditions. Thefinished-strips manufactured by the methods according to this inventionhave the mean “r” values not lower than 1.4 and the x-ray intensityratios not lower than 2 that provide excellent workability.

If manufacturing conditions are outside the range according to thepresent invention, appropriate precipitates are not formed in themanufacturing process. As a consequence, the preferable x-ray intensityand “r” value are not obtained in cold-rolled annealed steel strip.

The thicknesses of slabs and hot-rolled strips can be chosenappropriately. The reduction ratio, roll surface roughness, rolldiameter, rolling oil, rolling passes, rolling speed and rollingtemperature in cold-rolling can also be appropriately chosen.

Employment of a double rolling method, which applies intermediateannealing midway through cold-rolling further improves the properties offinished steel strip. Intermediate and final annealing can be appliedeither by bright annealing, which is implemented in non-oxidizingatmosphere such as hydrogen or nitrogen gas, or by annealing in theatmosphere.

INDUSTRIAL APPLICABILITY

The exemplary embodiments of the present invention efficiently providesCr-bearing heat-resistant steel strip having an excellent workabilitywithout requiring any new facilities. Accordingly, these exemplaryembodiments of the present invention provide a great industrialapplicability.

1. Cr-bearing heat-resistant steel sheet portion with a particularworkability comprising, in mass %: C of 0.001% to 0.010%, Si of 0.01% to0.60%, Mn of 0.05% to 0.6%, P of 0.01% to 0.04%, S of 0.0005% to0.0100%, Cr of 14% to 19%, N of 0.001% to 0.020%, Nb of 0.3% to 1.0%, Moof 0.5% to 2.0%, with a reminder comprising iron and unavoidableimpurities; and an x-ray intensity ration {111}/({100}+{211}) of atleast 2 in a central region of thickness of the portion.
 2. TheCr-bearing heat-resistant steel sheet according to claim 1, furthercomprising, in mass %, at least one of Cu of 0.5% to 3.0%, W of 0.01% to1.0%, and Sn of 0.01% to 1.00%.
 3. The Cr-bearing heat-resistant steelsheet according to claim 1, further comprising, in mass %, at least oneof Ti of 0.01% to 0.2%, Al of 0.005% to 0.100%, Mg of 0.0002% to0.0100%, and B of 0.0003% to 0.001%.
 4. Cr-bearing heat-resistant steelsheet portion with a particular workability comprising, in mass %: C of0.001% to 0.010%, Si of 0.01% to 0.60%, Mn of 0.05% to 0.6%, P of 0.01%to 0.04%, 5 of 0.0005% to 0.0100%, Cr of 14% to 19%, N of 0.001% to0.020%, Nb of 0.53% to 1.0%, Mo of 0.5% to 2.0%, with a remindercomprising iron and unavoidable impurities; and an x-ray intensityration {111}/({100}+{211}) of at least 2 in a central region ofthickness of the portion.
 5. The Cr-bearing heat-resistant steel sheetaccording to claim 1, further comprising, in mass %, at least one of Cuof 0.5% to 3.0%, W of 0.01% to 1.0%, and Sn of 0.01% to 1.00%.
 6. TheCr-bearing heat-resistant steel sheet according to claim 1, furthercomprising, in mass %, at least one of Ti of 0.01% to 0.2%, Al of 0.005%to 0.100%, Mg of 0.0002% to 0.0 100%, and B of 0.0003% to 0.001%.